Generally, mortar and concrete used for construction industries consist of a binder, water, and aggregate. As such, a typical inorganic binder, namely, cement (or cement clinker) consumes immense energy in the course of thermally treating limestone composed mainly of CaCO3 in the manufacturing process, and a large amount of CO2 gas is generated to the extent of 44 wt % or more of the amount of manufactured cement. The CO2 gas thus generated corresponds to about 7% of the total worldwide greenhouse gas emissions.
Specifically, cement is powder obtained by mixing silica, alumina, and lime as main components in an appropriate ratio, partially melting and sintering the mixture thus obtaining clinker, adding an appropriate amount of plaster to the clinker, and grinding the clinker. The manufacture of the cement clinker requires melting at a high temperature of about 1450° C., undesirably consuming a large amount of energy (about 30˜35 l/ton of oil). As well, upon manufacturing 1 ton of cement, about 700˜870 kg of CO2 is known to be emitted by just the chemical reaction of limestone and silicic acid.
The restriction placed on emissions of the greenhouse gas CO2 is regarded as very important in the field of cement manufacturing. Depending on the CO2 emission standards set by cement manufacturers, it is thought that the production of cement clinker will inevitably be reduced in the future. On the other hand, the demand for cement around the world is expected to increase by about 2.5˜5.8% per year up to the early 21st century, and thus, in order to satisfy an observance of the Kyoto protocol and an increase in the demand for cement, the development of a novel inorganic binder that reduces CO2 emission or does not emit CO2 is urgent.
In this regard, thorough research into alkali-activated binders without the use of cement, in order to substitute for cement, is ongoing. In particular, Korean Patent Application No. 2007-65185 discloses an alkali-activated binder comprising blast furnace slag and an inorganic alkaline material including a sodium-based material, in which the inorganic alkaline material includes one or more of sodium silicate and liquid type water glass, and the weight ratio of sodium-based material contained in the inorganic alkaline material to blast furnace slag is 0.038˜0.088, and the weight of sodium-based material is determined by being converted into Na2O.
Cement-free alkali-activated binders known to date including the above patent may be used in lieu of conventional common Portland cement, thus solving the problem of the disposal of industrial waste, thereby reducing the environmental load. Moreover, upon manufacturing the binder, energy can be saved and CO2 emissions can be prevented, thus exhibiting environmentally friendly properties. However, because the inorganic alkaline material used for the conventional alkali-activated binder contains sodium, when such a binder is used, it is difficult to control the total amount of alkali which should be restricted to inhibit the alkali-aggregate reaction in concrete. Specifically, in the inhibition of the alkali-aggregate reaction, the equivalent of Na2O per 1 m3 of concrete should be 0.3 kg or less. If the total amount of alkali falls outside of the restriction range, drastic slump loss may occur, undesirably deteriorating the quality of concrete.
On the other hand, known conventional loess paving materials include loess and cement which are mixed, and thus for them the above problems are unchanged because cement is used.
Also, drying shrinkage of concrete is affected by the amount of water used, the amount of binder, the fineness of binder, and the amount of aggregate, and may increase in proportion to increases in the amount of water used, the amount of binder and the fineness of binder and a decrease in the amount of aggregate. In particular, upon manufacturing concrete using loess, large drying shrinkage of loess must be controlled.
Road paving causes high drying shrinkage cracking and requires high tensile strength and flexural strength, and, in particular, surface drying shrinkage cracking imposes a poor outer appearance rather than structural problems, undesirably incurring distrust in builders and requiring repair of defects.
Furthermore, conventional loess paving materials include cement and loess, making it difficult to ensure necessary slump due to an increase in specific surface area in terms of properties of loess containing minute particles unlike sand. Moreover, in order to control drying shrinkage cracking depending on an increase in unit-water, a dry process is considered optimal. However, the dry process makes it difficult to manage the water content of the material. If the materials are not completely mixed, uniform quality cannot be ensured. Also, it is difficult to construct a paving material due to low slump, and pressure is applied using various devices, undesirably increasing the construction cost.
Therefore, there is a need to develop a novel alkali-activated binder, which is not influenced by the restriction of the total amount of alkali, may exhibit more stable workability and strength and is inexpensive thus reducing the production cost, and a novel loess paving material including the same.